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The long-term durability and safety of load-bearing joints are of great importance for the usability of bonded joints in structural applications. The probability of failure of a bonded component after a certain period …
The long-term durability and safety of load-bearing joints are of great importance for the usability of bonded joints in structural applications. The probability of failure of a bonded component after a certain period of time can be influenced by various ageing effects. The ageing of a bonded joint is defined as a change in strength or safety as a function of time caused by external factors (e.g. mechanical stress, temperature, humidity). Time-accelerated ageing tests are based on the intensification of damaging influencing factors in order to reduce both the time and the effort required to assess the durability of a component.
The correlation of results from accelerated ageing tests with long-term ageing under service conditions often remains an unresolved challenge. In this project, different commercially available computer-aided methods for non-linear regression analyses were used to predict the reliability and service life of bonded joints based on the results from the time-lapse ageing tests. Various physico-chemical models known from the literature were examined with regard to their suitability for modelling the ageing progression caused by two or more influencing factors. The adhesives used in this project include both a single-component, heat-curing epoxy resin-based adhesive and two two-component polyurethane adhesives. Using different sample geometries, results were generated with stainless steel and hot-dip galvanised sheet steel with and without cathodic dip coating. The ageing behaviour was modelled using combined functions based on the EYRING and PECK models, both of which proved to be suitable for describing the ageing behaviour of the tested joints. The safety prediction of bonded joints was carried out based on the probability of failure and the safety index using the EYRING model, as this proved to be more conservative.
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P 1091 - Development of a concept for the determination of prestressing force losses in prestressed bolted connections under fatigue loading Bolted connections used in structures subject to dynamic loads are specifically pretensioned in order to increase the load-bearing capacity and stiffness of the connections. Furthermore, pre-tensioning serves to achieve a significant increase in fatigue strength under tensile and/or shear stress. For these reasons, pretensioned bolted connections are always used in constructions subject to dynamic loads. Ideally, the preload losses that already occur in the bolted connections after assembly should be realistically estimated and implicitly taken into account when designing the structure so that the calculated preload force remains in the connection during the service life of the structure. In this research project, systematic investigations were carried out on prestressed bolted connections of categories B/C and E in accordance with EN 1993-1-8. The prestressing force losses were recorded separately according to their cause with the aid of settlement/relaxation tests as well as quasi-static and vibration tests and transferred to the planned service life by means of logarithmic extrapolation. The tests were carried out on M20 bolt assemblies with different fasteners/tightening methods as well as clamping length ratios and surface finishes of the test parts. The test results obtained made it possible to critically evaluate existing regulations. In view of the experimentally determined system reserves and the preload force losses, it was found that the use of direct force indicators is more likely to be recommended in connections that are preloaded for serviceability reasons due to high preload force losses. The operational pretensioning force losses in most of the analysed category E connections with a clamping length ratio (lk/d) of approximately 5 are in a range of up to 10 % and therefore fulfil the possible pretensioning force loss of no more than 10 % as specified in EN 1090-2 (no consideration of the pretensioning force loss). more than 10 % (no consideration of lockbolts). The connections with lk/d of approximately 2 exhibit preload force losses of more than 10 %, irrespective of the category of the connection and the surface finish. Even comparatively low coating thicknesses in the joint area lead to prestressing force losses of approx. 20 % over the service life of the structure. The combined prestressing method proves to be the only tightening method that has sufficient system reserves for a drop in prestressing force, even as a result of partial coating of the joints, without the prestressing force remaining over the service life of the structure falling below the design value. The present test results have created a fundamental starting point for the future consideration of prestressing force losses in design and dimensioning. FOSTA - Forschungsvereinigung Stahlanwendung e. V. Publication: April 2019 Research project P 1091 / IGF No. 18711 BG: Development of a concept for recording preload force losses in preloaded bolted joints under fatigue loading Institute for Metal and Lightweight Construction, University of Duisburg-Essen Univ.-Prof. Dr.-Ing. habil. Natalie Stranghöner Lukas Makevicius M.Sc. Fraunhofer Research Institution for Large Structures in Production Engineering IGP Univ.-Prof. Dr.-Ing. Wilko Flügge Univ.-Prof. Dr.-Ing. habil. Knuth-Michael Henkel Prof. Dr.-Ing. Ralf Glienke Dipl.-Ing. Maik Dörre Responsible for FOSTA - Forschungsvereinigung Stahlanwendung e. V. Dr Gregor Nüsse M.Sc.
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P 1092 - Quality assurance for electromagnetically pulsed welded joints (QuaSiMPS) As part of the project, investigations were carried out to assess the weldability of a high-strength aluminium alloy EN-AW-6016-T6 with dual-phase steel HCT780X using magnetic pulse welding. The process window for producing perfect welded joints with this combination of materials was created using numerous series of tests and validated by destructive testing. The influence of welding process parameters on the welding result was documented. The knowledge gained from this could be transferred to functional samples close to the application. Possible irregularities in the joining of mixed joints (aluminium-steel) using MPS technology were analysed and characterised. The results were summarised in the form of a defect catalogue based on DIN EN ISO 6520-2. The location of the respective irregularities in relation to the joining surface was described schematically and illustrated by an example using metallographic examination (cross-section). The suitability of inductively excited thermography as a non-destructive testing method for assessing the quality of an MPS joint was investigated and proven as part of the project. The methods required for reproducible defect introduction were developed and presented, and the test specimens produced were subjected to thermographic testing. A distinction was made between fault-free and faulty MPS joints. The findings of defect detection with the aid of active thermography could also be transferred to application-oriented, magnetic pulse welded functional samples. The characteristics of the joining surface in MPS and its influence on the strength of the welded joint were analysed. The correlation between the effective weld seam area and the maximum bearable tensile shear load was demonstrated. The size of the joint area can therefore be used to assess the quality of an MPS joint. Active thermography was used to determine the weld seam area of a lap-welded MPS joint with a measurement duration of approx. 0.2 seconds. In order to increase reproducibility and minimise the time required for evaluation, algorithms were developed that enable automated calculation of the joint area using both the microscopy images (evaluation after metallographic examination) and the thermograms. The functionality of the algorithms was validated using the results of the destructive testing and the accuracy was determined. The use of a transient thermal FEM analysis was investigated to further optimise the evaluation methodology of thermographic examinations. Various FEM models and approaches for defining realistic boundary conditions were discussed. A high level of agreement in the qualitative comparison between thermographic analyses and the simulation results demonstrated the enormous potential of this approach. MPS technology provides the user with a welding process that enables high-quality joining of challenging, dissimilar material combinations such as high-strength aluminium alloys with dual-phase steels. Induction-excited thermography can be used for this welding process as a non-destructive testing method to assess the quality of an MPS joint. In addition to detecting weld seam irregularities, it allows the weld seam surface to be determined non-destructively. Publication: 2020 Authors: I. Kryukov
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P 1094 - Data- and prediction-based generation of model parameters for the crash simulation of mechanically joined joints Due to the increasingly focussed implementation of innovative lightweight construction concepts, mechanical joining processes (semi-tubular self-pierce riveting, solid self-pierce riveting, flow-piercing bolting, lockbolting and high-speed bolting) are increasingly being used in multi-material constructions. These material combinations and mechanical joining processes can be subjected to high loads (crash load case). For the crash simulation of new types of mechanically joined connections, however, complete models for predicting the load-bearing behaviour in the crash load case are still lacking. Furthermore, a large number of practical experiments and measurements for model parameter identification and calibration of the equivalent models are necessary for the equivalent modelling of joints of entire vehicle structures. This is unrealistic due to the variety of joint pairings in a vehicle structure, and it would also be time-consuming and cost-intensive. The aim of the research project was, on the one hand, to develop a prediction functionality for determining characteristic values for unknown joints for all mechanical joining processes currently in use, which, together with experimentally determined data, serve as a data basis for an FE replacement model for the crash simulation of mechanically joined joints. On the other hand, the existing FE replacement model for crash simulation was adapted to other mechanical joining methods. Furthermore, a procedure was developed to provide an automated calibration of the input parameters for an FE substitute model for crash simulation from the predicted and measured data. The validation of the prediction results and the FE replacement models for crash simulation was carried out by means of tests and simulations at sample and specimen component level. By creating a database and the possibility of predicting characteristic values of mechanically joined connections that have not yet been measured in reality, as well as FE substitute modelling for the crash simulation for the above-mentioned mechanically joined connections, connection failure can be mapped in the crash simulation as early as the design phase and its effects evaluated. This enables users, designers and calculation engineers to optimise components and increase their performance. With a forecast and automated generation of model parameters for the calculation of an FE replacement model, this can be carried out in a time and cost-saving manner without first having to carry out lengthy and expensive series of tests and simulations to determine the parameters. The IGF project 18468 BG of the Gesellschaft zur Förderung angewandter Informatik e. V., Berlin and the FOSTA - Forschungsvereinigung Stahlanwendung e. V., Düsseldorf, as a cooperating research association, was funded by the Federal Ministry for Economic Affairs and Energy via the AiF as part of the programme for the promotion of joint industrial research (IGF) on the basis of a resolution of the German Bundestag. The project was carried out by the Gesellschaft zur Förderung angewandter Informatik e. V., Berlin, the Fraunhofer Institute for Mechanics of Materials IWM, Freiburg and the Laboratory for Materials and Joining Technology (LWF) at Paderborn University. Society for the Promotion of Applied Computer Science e. V., Berlin Dipl.-Inf. Daniel Herfert M. Sc. Matthias Günther Fraunhofer Institute for Mechanics of Materials IWM, Freiburg Dr Silke Sommer M. Sc. Philip Rochel Laboratory for Materials and Joining Technology (LWF), University of Paderborn Prof. Dr.-Ing. Gerson Meschut Dr David Hein M. Sc. Patrick Giese Responsible for FOSTA - Forschungsvereinigung Stahlanwendung e. V. Dipl.-Ing. Franz-Josef Heise
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P 1097 - Development of an integrated floor system for steel and composite construction The aim of the research project is the development of an integrated floor system for steel and composite construction and the derivation of engineering models to describe the floor load-bearing behaviour. The starting point is the InaDeck floor system, which was designed as part of the FOSTA research project P 879. This is characterised by large spans and the building technology integrated in the ceiling void. In order to bring the ceiling system to demonstrator maturity, the individual ceiling elements must be optimised in terms of load-bearing capacity, cost-effectiveness and resource consumption (rolled profiles and composite materials), and the integration of the ceiling system into the overall structure must be developed. In this research project, possible system and cross-section variations that allow for a flexibilisation of the grid dimension are first examined. Based on this, existing steel connection constructions to columns and alternating beams are analysed and adapted for the ceiling system. For the friction-locked joints between the floor elements, grouted joints as well as detachable and theoretical analyses are investigated. For this purpose, the load-bearing and load-bearing capacity showed a decrease in the load-bearing capacity with increasing tensile load. A similar effect was also observed in the tests in cracked concrete. In the shear tests with additional transverse bending, a reduction in the load-bearing capacity was also observed, but this was not confirmed in the girder tests. In final girder tests, the load-bearing mechanisms in the web opening area under positive and negative moment loading on the one hand and the influences of combined transverse and longitudinal bending loading in the floor element on the other were investigated. The vibration behaviour of the lightweight composite floor is analysed on a 10.5 m long composite floor element with an increasing number of web openings. This showed a positive effect on the natural frequency with web openings in the support area. The damping of the floor was consistently low. Engineering models for the floor system were derived on the basis of the investigations carried out. The result of the research is an integrated steel composite floor system for modular, long-span load-bearing structures that is ready for dismantling. Publication date: January 2021 Authors: Dipl.-Ing. M. Kopp, Prof. Dr.-Ing. M. Feldmann, Dr.-Ing. M. Claßen, Prof. Dr.-Ing. J. Hegger
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